Led module and method for fabricating the same

ABSTRACT

Disclosed is a method for fabricating an LED module. The method includes: constructing a chip-on-carrier including a chip retainer having a horizontal bonding plane and a plurality of LED chips in which electrode pads are bonded to the bonding plane of the chip retainer; and transferring the plurality of LED chips in a predetermined arrangement from the chip retainer to a substrate by transfer printing. The transfer printing includes: primarily section-wise exposing a transfer tape to reduce the adhesive strength of the transfer tape such that bonding areas are formed at predetermined intervals on the transfer tape; and pressurizing the transfer tape against the LED chips on the chip retainer to attach the LED chips to the corresponding bonding areas of the transfer tape and detaching the electrode pads of the LED chips from the chip retainer to pick up the chips.

CROSS-REFERENCE TO RELATED PATENT APPLICATION(S)

This application is a division of U.S. patent application Ser. No.15/793,030, filed Oct. 25, 2017, which is a division of U.S. patentapplication Ser. No. 15/558,192, filed Sep. 13, 2017, which is the U.S.National Stage entry of PCT/KR2017/008269, filed Aug. 1, 2017, whichclaims priority to Korean Patent Application No. 10-2016-0102239, filedAug. 11, 2016, Korean Patent Application No. 10-2016-0157045, filed Nov.24, 2016, Korean Patent Application No. 10-2017-0030395, filed on Mar.10, 2017, Korean Patent Application No. 10-2017-0032900, filed Mar. 16,2017, and Korean Patent Application No. 10-2017-0032955, filed Mar. 16,2017, the entire contents of which are incorporated by reference intheir entirety.

TECHNICAL FIELD

The present invention relates to an LED module and a method forfabricating the same. More specifically, the present invention relatesto a method for fabricating an LED module by using transfer printing andflip bonding.

BACKGROUND ART

Full-color LED displays in which LEDs emitting light at differentwavelengths are grouped into pixels have been proposed as potentialreplacements for displays using LEDs as backlight light sources. Eachpixel consists of red, green, and blue LEDs or red, green, blue, andwhite LEDs. In such an LED display, red, green, and blue LEDs arefabricated in packages and are mounted on a substrate. However, due tothe large distances between the constituent LEDs of each pixel,high-quality resolution is difficult to obtain. Pixels consisting ofpackages of LEDs are difficult to apply to micro-LED displays that haverecently received much attention. LED pixel units have also beenproposed in which red LEDs, green LEDs, and blue LEDs constituting onepixel are mounted in one package. In such an LED pixel unit, thedistance between the adjacent LEDs (i.e. sub-pixels) in one pixel issmall but the distance between the adjacent pixels is difficult toreduce. Further, light interference may occur between the red, green,and blue LEDs.

Thus, for the purpose of reducing the distance between pixels, thepresent inventors have attempted to fabricate an LED display module inwhich groups of LED chips, each of which includes red LED, green LED,and blue LED chips, are arrayed in a matrix on a PCB substrate. It is,however, difficult to mount the LED chips at predetermined heights andintervals on the micrometer-sized substrate. Different heights and/orintervals between the LED chips mounted on the substrate deteriorate thecolor reproducibility of the LED display module. Wire bonding isnecessary for electrical connection between electrode pads and the LEDchips on the substrate but it takes at least tens to hundreds of hoursto manufacture one product. Particularly, in the course of mounting tensto hundreds of LED chips on the substrate, some of the LED chips are notaccurately located at desired positions, making it impossible to achievea designed light emitting pattern and causing serious color deviation.Particularly, techniques for fabricating LED display modules includingarraying micro-LEDs with a size of several to several hundreds ofmicrometers on an active matrix (AM) substrate are currently in use butare difficult to apply to the fabrication of display modules with highprecision and good quality based on conventional chip mountingtechnology.

SUMMARY

In this connection, a technique for transferring LED chips arrayed atgiven positions to a substrate by total transfer printing can provide asolution to the problems of the prior art. However, although LED chipstogether with the overlying electrode pads are transferred to andarrayed on a substrate by total transfer printing, an additionalprocess, such as wire bonding, is required. The increased number ofprocessing steps leads to an increase in fabrication cost. Further, thealignment of LED chips is disordered, causing considerable deteriorationof quality. In contrast, transfer printing of flip-bonded LED chipsincluding downwardly arranged electrode pads to a substrate can avoidthe need of an additional process, such as wire bonding, and as aresult, the alignment of the LED chips can be prevented from beingdisordered. In addition, transfer printing enables an array of selectedLED chips with a size of tens to hundreds of micrometers in a desiredarrangement.

Therefore, the present invention is directed to providing a method forfabricating an LED module by transferring LED chips from a chip retaineradapted to retain the LED chips in a predetermined arrangement withoutbeing disordered to a substrate by transfer printing and subsequent flipbonding of the LED chips on the substrate.

TECHNICAL SOLUTION

A method for fabricating an LED module according to one aspect of thepresent invention includes: constructing a chip-on-carrier including achip retainer having a horizontal bonding plane and a plurality of LEDchips in which electrode pads are bonded to the bonding plane of thechip retainer; and transferring the plurality of LED chips in apredetermined arrangement from the chip retainer to a substrate bytransfer printing, wherein the transfer printing includes: primarilysection-wise exposing a transfer tape to reduce the adhesive strength ofthe transfer tape such that bonding areas are formed at predeterminedintervals on the transfer tape; and pressurizing the transfer tapeagainst the LED chips on the chip retainer to attach the LED chips tothe corresponding bonding areas of the transfer tape and detaching theelectrode pads of the LED chips from the chip retainer to pick up thechips.

According to one embodiment, the primary exposure includes exposing thetransfer tape through a photomask formed with a plurality oflight-transmitting windows.

According to one embodiment, the method further includes, after the chippick-up, secondarily exposing the transfer tape attached with the LEDchips to weaken the adhesive strength of the transfer tape as a wholeand placing the plurality of LED chips from the transfer tape whoseadhesive strength is weakened as a whole on the substrate.

According to one embodiment, the chip pick-up includes pressurizing thetransfer tape against the LED chips bonded onto the chip retainer with apick-up roller rolling in one direction.

According to one embodiment, the placing includes pressurizing the LEDchips attached to the transfer tape against the substrate with a placingroller so that the electrode pads of the LED chips are attached to pairsof bumps formed on the substrate.

According to one embodiment, the adhesive strength of the transfer tapeduring the placing is lower than the adhesive strength of an adhesiveloaded on the pairs of bumps.

According to one embodiment, the chip-on-carrier construction includespreparing a chip retainer having a horizontal bonding plane, preparing aplurality of LED chips, and attaching the LED chips onto the bondingplane to form one or more LED chip arrays wherein the preparation of aplurality of LED chips includes preparing a plurality of LED chipsincluding downwardly extending n-type electrode pads and p-typeelectrode pads and the chip attachment includes directly bonding then-type electrode pads and the p-type electrode pads to the bondingplane.

According to one embodiment, the chip attachment includes attaching theplurality of LED chips to the bonding plane such that the pitch in theLED chip arrays on the chip retainer is one-nth (where n is a naturalnumber equal to or greater than 1) of that in the LED chip arraystransferred to the substrate by the transfer printing, the pitchrepresenting the horizontal distance between the center of one LED chipand the center of the adjacent LED chip.

An LED module according to a further aspect of the present inventionincludes: a plurality of arrayed LED chips transferred from an externalchip retainer, each of the LED chips having electrode pads at one sidethereof and a plane attached to a transfer tape at the other sidethereof; and a substrate having a plurality of bumps flip-bonded to theelectrode pads, wherein the transfer tape is divided into exposed areasand unexposed areas by primary light irradiated from the outside ontothe side opposite to the LED chips and the LED chips are bonded to andpicked up on the unexposed areas.

According to one embodiment, the transfer tape loses its adhesivestrength in the bonding areas by secondary light irradiated from theoutside onto the side opposite to the LED chips and is detached from theLED chips.

According to one embodiment, the bonding areas of the transfer tape areprotected from the primary light irradiated from the outside onto theside opposite to the LED chips through a photomask.

According to one embodiment, the LED chips are picked up by a rollerthat rolls while pressurizing the transfer tape against the LED chips.

According to one embodiment, the LED chips are situated and aligned atdesired positions on the substrate and are then detached from thetransfer tape by the pressurization of the rolling roller.

According to one embodiment, the chip retainer includes a horizontalbonding plane and the electrode pads extending downwardly at one side ofeach of the LED chips are directly bonded to the chip retainer.

According to one embodiment, the plurality of LED chips have the sameheight from the bonding plane of the chip retainer.

According to one embodiment, the pitch in the LED chip arrays on thechip retainer is one-nth (where n is a natural number equal to orgreater than 1) of that in the LED chip arrays transferred to thesubstrate, the pitch representing the horizontal distance between thecenter of one LED chip and the center of the adjacent LED chip.

According to one embodiment, the adhesive strength of the chip retaineris lower than the adhesive strength of the unexposed areas of thetransfer tape and is higher than the adhesive strength of the exposedareas of the transfer tape.

According to one embodiment, the plurality of LED chips arrayed on thechip retainer consist of only one type of LED chip selected from red,green, and blue LED chips produced by the same process.

According to one embodiment, the plurality of LED chips arrayed on thechip retainer include red LED chips, green LED chips, and blue LEDchips.

According to one embodiment, the chip retainer may be a flexible film.

Advantageous Effects

According to the present invention, the LED module can be fabricated byprecisely aligning a plurality of LED chips on a substrate by transferprinting. According to the transfer printing, all or some LED chipsarrayed at given positions are arrayed in a desired arrangement on atarget substrate.

Other effects of the present invention will be better understood fromthe following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flow chart for explaining a method for fabricating an LEDmodule according to one embodiment of the present invention;

FIG. 2 illustrates a transfer printing process in a method forfabricating an LED module according to one embodiment of the presentinvention;

FIG. 3 illustrates a process for constructing a chip-on-carrierincluding blue LED chips in a method for fabricating an LED moduleaccording to one embodiment of the present invention;

FIG. 4 illustrates a process for constructing a chip-on-carrierincluding green LED chips in a method for fabricating an LED moduleaccording to one embodiment of the present invention;

FIG. 5 illustrates a process for constructing a chip-on-carrierincluding red LED chips in a method for fabricating an LED moduleaccording to one embodiment of the present invention;

FIG. 6 illustrates a process for constructing a chip-on-carrierincluding red, green, and blue LED chips in accordance with analternative embodiment of the present invention;

FIG. 7 is a plan view illustrating a substrate of a display moduleaccording to a further embodiment of the present invention and arrays ofelectrode patterns arranged in a matrix on the substrate;

FIG. 8 is a plan view illustrating a display module including groups ofLED chips arranged in a matrix on the electrode patterns illustrated inFIG. 7;

FIG. 9 is a cross-sectional view for explaining the groups of LED chipsillustrated in FIG. 8;

FIG. 10 is a plan view for explaining an alternative embodiment of theLED module illustrated in FIGS. 7 to 9;

FIG. 11 is a flow chart for explaining a method for fabricating the LEDmodule illustrated in FIGS. 7 to 10;

FIG. 12 illustrates a procedure for transfer printing of LED chips on asubstrate for the fabrication of the LED module illustrated in FIGS. 7to 10;

FIG. 13 is a flow chart for explaining a method for fabricating an LEDmodule by using selective transfer printing according to anotherembodiment of the present invention;

FIG. 14 illustrates the step of picking up chips in the methodillustrated in FIG. 13;

FIG. 15 is an enlarged view of circle “A” of FIG. 14;

FIG. 16 illustrates the step of moving chips and the step of weakeningthe adhesive strength of a carrier tape just before placing of the chipsin the method illustrated in FIG. 15;

FIG. 17 illustrates the step of placing chips in the method illustratedin FIG. 16;

FIG. 18 is an enlarged view of circle “B” of FIG. 17;

FIG. 19 illustrates a process for picking up LED chips by total transferprinting; and

FIGS. 20a and 20b illustrate several exemplary processes for picking upLED chips by selective transfer printing.

DETAILED DESCRIPTION

[Fabrication of First Type LED Module]

Referring to FIGS. 1 and 2, a method for fabricating an LED moduleaccording to one embodiment of the present invention includesconstructing a chip-on-carrier including a chip retainer 2 having ahorizontal bonding plane and a plurality of LED chips 1 bonded to thebonding plane of the chip retainer 2 (S1) and transferring the LED chips1 in a predetermined arrangement from the chip retainer 2 to a substrate5 by transfer printing (S2). In S1, a chip-on-carrier (COC) isconstructed. The chip-on-carrier a plurality of LED chips includingelectrode pads 122 and 142 (i.e. n-type electrode pads 122 and p-typeelectrode pads 142) retained on the chip retainer 2. The pitch P betweenthe LED chips 1 in the chip-on-carrier (COC) is one-nth (where n is anatural number equal to or greater than 1) of that between the LED chipsin the arrays transferred to the substrate by the transfer printing. Asused herein, there term “pitch” is defined as the horizontal distancebetween the center of one LED chip and the center of the adjacent LEDchip.

S2 includes transfer printing the plurality of LED chips 1 attached ontothe chip retainer 2 of the chip-on-carrier (COC) on a substrate 5.

S2 includes primarily section-wise exposing a transfer tape 3 to reducethe adhesive strength of the transfer tape such that bonding areas areformed at predetermined intervals on the transfer tape (S21),pressurizing the transfer tape 3 against the LED chips 1 on thechip-on-carrier COC to attach the LED chips 1 to the correspondingbonding areas 32 of the transfer tape 3 (chip pick-up, S22), secondarilyexposing the transfer tape 3 to weaken the adhesive strength of thetransfer tape 3 as a whole (S23), and pressurizing the LED chips 1attached to the transfer tape 3 whose adhesive strength is weakenedagainst the substrate 5 (placing down, S24). After S24, the transfertape 3 is detached from the LED chips 1 (S25).

First, in S21, a photomask 220 including a plurality of UVlight-transmitting windows 222 is arranged. UV light from a first UVlight source 230 is irradiated onto the transfer tape 3 through the UVlight-transmitting windows 222. In other words, a transfer tape 3 isexposed to UV light passing through UV light-transmitting windows 222from a first UV light source 230. This exposure weakens the adhesivestrength of areas of the transfer tape 3 exposed to the UV light. Onlyareas other than the exposed areas whose adhesive strength is weakenedare bonding areas 32, which are arranged at regular intervals.

Next, in S22, the transfer tape 3 is pressurized against the LED chips 1preliminarily attached onto the chip retainer 2 with a pick-up roller240 that rolls in one direction.

In S23, the transfer tape 3 carrying the LED chips 1 is exposed to UVlight. The UV exposure weakens the adhesive strength of the bondingareas 32 of the transfer tape 3. The second UV light source 260irradiates UV light onto the transfer tape 3 to weaken the adhesivestrength of all areas (including the bonding areas 32 where the LEDchips 1 are retained) of the transfer tape 3.

S24 includes pressurizing the LED chips 1 bonded to the transfer tape 3whose adhesive strength is weakened against a substrate 5.

The adhesive strength of the transfer tape 3 after S23 is lower thanthat of an adhesive loaded on the pairs of bumps 5 a and 5 b. Theplacing roller 270 rolls and pressurizes the LED chips 1 attached to thetransfer tape 3 against the substrate 5, more specifically the pairs ofbumps 5 a and 5 b. As a result of the pressurization, the correspondingLED chips 1 are attached onto the substrate 5. The placing roller 270may be provided with a flexible blanket on the outer circumference of aroller body coupled to a shaft. The provision of the blanket allows theLED chips 1 to be better placed down during rolling and can protect theLED chips 1 from damage caused by pressurization during rolling. Asecond stage 290 may ascend to help the placing roller 270 pressurizethe LED chips. Next, the transfer tape 3 is detached from the LED chips1 (S25). Then, the LED chips 1 placed down on the substrate 5 can bebonded onto the substrate by subsequent reflow soldering.

Referring to FIG. 3, the process for constructing a chip-on-carrierincluding blue LED chips 1B includes: producing a blue LED wafer WBincluding a sapphire substrate 10B and a nitride gallium epilayer grownon the sapphire substrate 10B and including an n-type semiconductorlayer 12B, an active layer 13B, and a p-type semiconductor layer 14B(S11-B); patterning the epilayer such that a plurality of blue lightemitting cells CB including all exposed areas of the p-typesemiconductor layer 14B and the n-type semiconductor layer 12B, whichare stepped with each other, at the side opposite to the sapphiresubstrate 10B are arranged in a matrix (S12-B); forming p-type electrodepads 142B in the exposed area of the p-type semiconductor layer 14B andforming n-type electrode pads 122B in the exposed area of the n-typesemiconductor layer 12B (S13-B); singulating the LED wafer WB into lightemitting cell (CB) units to produce a plurality of blue LED chips 1Bincluding the sapphire substrate 10B, the wafer layers 12B, 13B, and 14Bon the sapphire substrate 10B, and the p-type electrode pads 142B andthe n-type electrode pads 122B formed on the wafer layers 12B, 13B, and14B opposite to the sapphire substrate 10B (S14-B); and inverting theplurality of blue LED chips 1B such that the p-type electrode pads 142Band the n-type electrode pads 122B are directed downward and attachingthe blue LED chips to the adhesive chip retainer 2 such that theplurality of blue LED chips 1B are arranged in a matrix (S15-B). Theadhesive strength of the chip retainer 2 is lower than that of thetransfer tape 3 before and after UV exposure.

In S11-B, an epilayer grows such that an active layer 13B includes anIn_(x)Ga(1-x)N well layer. The amount of In in the well layer isappropriately controlled to obtain blue LED chips 1B. In S12-B, aplurality of row valleys and a plurality of column valleys are formed inthe epilayer by etching. As a result of the etching, a plurality oflight emitting cells CB are formed, each of which includes an n-typesemiconductor layer 12B, the active layer 13B, and a p-typesemiconductor layer 14B on a sapphire substrate 10B or a latticematching layer 11B thereon. Next, the p-type semiconductor layer 14B andthe active layer 13B of each of the light emitting cells CB arepartially etched to expose the n-type semiconductor layer 12B. In S13-B,p-type electrode pads 142B are formed in the exposed area of the p-typesemiconductor layer 14B and n-type electrode pads 122B are formed in theexposed area of the n-type semiconductor layer 12B. The n-type electrodepads 122B and the p-type electrode pads 142B are formed to suchthicknesses to compensate for the step height such that the height fromthe bottom of the sapphire substrate 10B to the upper surfaces of then-type electrode pads 122B is equal to the distance from the bottom ofthe sapphire substrate 10B to the p-type electrode pads 142B. In S14-B,the blue LED wafer WB is singulated into the light emitting cell CBunits using a suitable cutting tool T, such as a blade or saw, or asuitable laser to produce a plurality of blue LED chips 1B. Until thisstep, the n-type electrode pads 122B and the p-type electrode pads 142Bare directed upward and the sapphire substrate 10B is directed downward.In S15-B, the plurality of blue LED chips 1B are inverted such that then-type electrode pads 122B and the p-type electrode pads 142B are bondedto a horizontal bonding plane of a chip retainer 2B to attach theplurality of blue LED chips 1B onto the chip retainer 2B. Here, theupper surface of each of the LED chips 1 becomes a base plane of thesapphire substrate 10B and the heights of all LED chips 1 from thebonding plane of the chip retainer 2B are constant. All blue LED chips1B of the chip-on-carrier thus produced are arranged in row and columnarrays on the chip retainer 2B. The chip retainer 2B is preferably aflexible chip retaining film having a horizontal bonding plane. Thepitch P between the blue LED chips 1B in a particular one-row arraybonded to and retained on the horizontal bonding plane of the chipretainer 2B is determined as one-nth (where n is a natural number equalto or greater than 1) of that between the blue LED chips in a one-rowarray arrayed on a substrate by transfer printing. Herein, the pitchbetween the chips is defined as the horizontal distance between thecenter of one LED chip and the adjacent LED chip.

Referring to FIG. 4, the process for constructing a chip-on-carrierincluding green LED chips 1G includes: producing a green LED wafer WGincluding a sapphire substrate 10G and a nitride gallium epilayer grownon the sapphire substrate 10G and including an n-type semiconductorlayer 12G, an active layer 13G, and a p-type semiconductor layer 14G(S11-G); patterning the epilayer such that a plurality of green lightemitting cells CG including all exposed areas of the p-typesemiconductor layer 14G and the n-type semiconductor layer 12G, whichare stepped with each other, at the side opposite to the sapphiresubstrate 10G are arranged in a matrix (S12-G); forming p-type electrodepads 142G in the exposed area of the p-type semiconductor layer 14G andforming n-type electrode pads 122G in the exposed area of the n-typesemiconductor layer 12G (S13-G); singulating the LED wafer WG into lightemitting cell (CB) units to produce a plurality of green LED chips 1Gincluding the sapphire substrate 10G, the wafer layers 12G, 13G, and 14Gon the sapphire substrate 10G, and the p-type electrode pads 142G andthe n-type electrode pads 122G formed on the wafer layers 12G, 13G, and14G opposite to the sapphire substrate 10G (S14-G); and inverting theplurality of green LED chips 1G such that the p-type electrode pads 142Gand the n-type electrode pads 122G are directed downward and attachingthe green LED chips to the adhesive chip retainer 2 such that theplurality of green LED chips 1G are arranged in a matrix (S15-G).

In S11-G, an epilayer grows such that an active layer 13G includes anIn_(x)Ga(1-x)N well layer. The content of In is adjusted to a higherlevel to obtain the green LED chips 1G than that used to obtain the blueLED chips 1B.

In S12-G, a plurality of row valleys and a plurality of column valleysare formed in the epilayer by etching. As a result of the etching, aplurality of light emitting cells CG are formed, each of which includesan n-type semiconductor layer 12G, the active layer 13G, and a p-typesemiconductor layer 14G on a sapphire substrate 10G or a latticematching layer 11G thereon. Next, the p-type semiconductor layer 14G andthe active layer 13G of each of the light emitting cells CG arepartially etched to expose the n-type semiconductor layer 12G. In S13-G,p-type electrode pads 142G are formed in the exposed area of the p-typesemiconductor layer 14G and n-type electrode pads 122G are formed in theexposed area of the n-type semiconductor layer 12G. The n-type electrodepads 122G and the p-type electrode pads 142G are formed to suchthicknesses to compensate for the step height such that the height fromthe bottom of the sapphire substrate 10G to the upper surface of then-type electrode pads 122G is equal to the distance from the bottom ofthe sapphire substrate 10G to the p-type electrode pads 142G. In S14-G,the green LED wafer WB is singulated into the light emitting cell CGunits using a suitable cutting tool T, such as a blade or saw, or asuitable laser to produce a plurality of green LED chips 1G. Until thisstep, the n-type electrode pads 122G and the p-type electrode pads 142Gare directed upward and the sapphire substrate 10G is directed downward.In S15-G, the plurality of green LED chips 1G are inverted such that then-type electrode pads 122G and the p-type electrode pads 142G are bondedto a horizontal bonding plane of a chip retainer 2G to attach theplurality of green LED chips 1G onto the chip retainer 2G. Here, theupper surface of each of the green LED chips 1G becomes a base plane ofthe sapphire substrate 10G and the heights of all green LED chips 1Gfrom the bonding plane of the chip retainer 2G are constant. All greenLED chips 1G of the chip-on-carrier thus produced are arranged in rowand column arrays on the chip retainer 2G. The chip retainer 2G ispreferably a flexible chip retaining film having a horizontal bondingplane. The pitch P between the green LED chips 1G in a particularone-row array bonded to and retained on the horizontal bonding plane ofthe chip retainer 2G is determined as one-nth (where n is a naturalnumber equal to or greater than 1) of that between the green LED chipsin a one-row array arrayed on a substrate by transfer printing.

Referring to FIG. 5, the process for constructing a chip-on-carrierincluding red LED chips includes: producing a red LED wafer WR includinga sapphire substrate 10R and an epilayer bonded onto the sapphiresubstrate 10R and including a p-type semiconductor layer 12R, an activelayer 13R, and an n-type semiconductor layer 14R (S11-R); patterning theepilayer such that a plurality of red light emitting cells CR, each ofwhich is divided into a first area a1 and a second area a2 by an exposedgroove 120R on the p-type semiconductor layer, are arranged in a matrix(S12-R); forming n-type electrode pads 142R connected to the n-typesemiconductor layer 14R on the n-type semiconductor layer 14R in thefirst area a1 and forming p-type electrode pads 122R electricallyisolated from the n-type semiconductor layer 14R by an insulating layerR and connected to the an interconnection layer L extending from theexposed groove 120R on the n-type semiconductor layer 14R in the secondarea a2 (S13-R); singulating the red LED wafer WR into light emittingcell (CB) units to produce a plurality of red LED chips 1R including thesapphire substrate 10R, the wafer layers on the sapphire substrate, andthe p-type electrode pads 122R and the n-type electrode pads 142R(S14-R); and inverting the plurality of red LED chips 1R such that thep-type electrode pads 122R and the n-type electrode pads 142R aredirected downward and attaching the red LED chips to the adhesive chipretainer 2R such that the plurality of red LED chips 1R are arranged ina matrix (S15-R).

S11-R includes: growing an epilayer including an n-type semiconductorlayer 14R, an active layer 13R, and a p-type semiconductor layer 12R ona GaAs substrate GS, the n-type semiconductor layer 14R including ann-AlGaInP layer and an n-cladding layer, the active layer 13R includingMQW, and the p-type semiconductor layer 12R including a p-cladding layerand a p-GaP layer; bonding a sapphire substrate 10R as a supportsubstrate to the p-type semiconductor layer 12R, a SiO₂ bonding layer11R being interposed between the substrate and the p-type semiconductorlayer; and detaching the GaAs substrate GS as a growth substrate fromthe n-type semiconductor layer 14R at the side opposite to the sapphiresubstrate.

In S12-R, a plurality of row valleys and a plurality of column valleysare formed in the epilayer by etching. As a result of the etching, aplurality of light emitting cells CR are formed, each of which includesthe p-type semiconductor layer 12R, the active layer 13R, and the n-typesemiconductor layer 14R on the sapphire substrate 10R or the bondinglayer 11R thereon. Next, each light emitting cell CR is etched to adepth reaching the p-GaP layer of the p-type semiconductor layer 12R toform an exposed grooves 120R on the p-type semiconductor layer such thatthe upper area of the light emitting cell CR includes a first area a1and a second area a2. In S13-R, n-type electrode pads 142R directlyconnected to the n-type semiconductor layer 14R are formed on the n-typesemiconductor layer 14R in the first area a1 and p-type electrode pads122R connected to the p-type semiconductor layer 12R through aninterconnection layer L extending to the exposed groove 120R on thep-type semiconductor layer are formed on the n-type semiconductor layer14R in the second area a2. The distance from the bottom of the sapphiresubstrate 10R to the n-type electrode pads 142R, i.e. the height of then-type electrode pads 142R from the bonding plane, is the same as thedistance from the bottom of the sapphire substrate 10R to the p-typeelectrode pads 122R, i.e. the height of the p-type electrode pads 122Rfrom the bonding plane. In S14-R, the LED wafer WR is singulated intothe light emitting cell CR units to produce a plurality of red LED chips1R. Until this step, the n-type electrode pads 142R and the p-typeelectrode pads 122R are directed upward and the sapphire substrate 10Ris directed downward. In S14-R, the plurality of red LED chips 1R areinverted such that the n-type electrode pads 142R and the p-typeelectrode pads 122R are bonded to the horizontal bonding plane of a chipretainer 2R to attach the plurality of red LED chips 1R onto the chipretainer 2R. Here, the upper surface of each of the LED chips 1R becomesa base plane of the sapphire substrate 10R and the heights of all LEDchips 1R from the bonding plane of the chip retainer 2R are constant.All red LED chips 1R of the chip-on-carrier thus produced are arrangedin row and column arrays on the chip retainer 2R.

As illustrated in FIG. 6, a chip-on-carrier may also be considered inwhich a red LED chip 1R, a green LED chip 1G, and a blue LED chip 1B aresequentially attached onto a chip retainer 2. The chip-on-carrier cantransfer the red LED chip 1R, the green LED chip 1G, and the blue LEDchip 1B to a substrate all at one time by transfer printing.

[Fabrication of Second Type LED Module]

Referring to FIGS. 7 to 9, an LED module 100 according to one embodimentof the present invention includes a rectangular substrate 110, aplurality of electrode patterns 130 having a predetermined heightarranged in a matrix on the substrate 110, and a plurality of groups ofLED chips 150 arranged in a matrix on the substrate 110 so as tocorrespond to the electrode patterns 130.

The electrode patterns 130 are formed on the substrate 110 and LED chips151, 153 or 155 are mounted thereon. When the substrate 110 is flat, theplurality of LED chips 151, 153 or 155 can be mounted to a predeterminedheight thereon. The type of the substrate 110 is not limited so long asit is flat. For example, the substrate 110 may be rigid or flexible.Specifically, the substrate 110 may be a rigid plastic, ceramic or glasssubstrate or a flexible substrate such as an FPCB.

As mentioned above, the plurality of electrode patterns 130 are disposedin a matrix on the substrate 110 and have the same height as a whole.For example, the plurality of electrode patterns 130 having apredetermined height may be formed by partially removing a conductivemetal film formed to a predetermined thickness on one flat surface ofthe substrate 110 by etching or forming a conductive metal film having apredetermined pattern to a predetermined height on one flat surface ofthe substrate 110 through a mask.

The plurality of electrode patterns 130 are arranged in a matrix withrows and columns in the widthwise and lengthwise directions,respectively. In the description of this embodiment, the widthwise andlengthwise directions are defined as directions parallel to line Lx andLy in FIGS. 7 and 8, respectively.

Each of the electrode patterns 130 includes a common electrode pad 139aligned to a first end line EL′ parallel to the lengthwise direction ofthe matrix, and a first individual electrode pad 131, a secondindividual electrode pad 132, and a third individual electrode pad 133aligned to a second end line EL parallel to the first end line EL′ andlocated between the first end line EL′ and the second end line EL.

The first individual electrode pad 131, the second individual electrodepad 132, and the third individual electrode pad 133 are individuallyconnected to an input power source (not illustrated) in a finishedelectric circuit and enable independent control of LED chips, which willbe explained in detail below.

The common electrode pad 139 includes two recesses depressed in asubstantially rectangular shape inwardly from the first end line EL′ andthree branches, i.e. a first branch 136, a second branch 137, and athird branch 138, whose shapes are defined by the two recesses. Each ofthe first branch 136, the second branch 137, and the third branch 138defines an area where a bonding bump formed at one side of an LED chipis bonded and contributes to the prevention of shorting caused byundesired distortion or slipping of the bonding bump upon flip-chipbonding of the LED chip.

The first individual electrode pad 131, the second individual electrodepad 132, and the third individual electrode pad 133 are disposed inparallel at uniform intervals such that LED chips emitting light atdifferent wavelengths, i.e. a first LED chip 151, a second LED chip 153,and a third LED chip 155, are arrayed in parallel at uniform intervals,which will be explained in detail below. Each of the first individualelectrode pad 131, the second individual electrode pad 132, and thethird individual electrode pad 133 defines an area where a bump isbonded. Each of the electrode pads is connected the corresponding LEDchip through the bump.

The first individual electrode pad 131, the second individual electrodepad 132, and the third individual electrode pad 133 include ends 131 b,132 b, and 133 b coinciding with the second end line EL, opposite ends131 c, 132 c, and 133 c close to the common electrode pad 139, andlateral sides 131 a, 132 a, and 133 a parallel to the widthwisedirection, respectively. The first individual electrode pad 131, thesecond individual electrode pad 132, and the third individual electrodepad 133 include narrow portions formed at the ends 131 b, 132 b, and 133b and wide portions formed at the ends 131 c, 132 c, and 133 b andhaving larger widths than the narrow portions, respectively. Thepositions where bumps are formed are limited to the wide portions.

The ends 131 c, 132 c, and 133 c of the individual electrode pads 131,132, and 133 are located parallel and close to ends 136′, 137′, and 138′of the branches 136, 137, and 138 of the common electrode pad 139coinciding with the first end line EL′, respectively.

As well illustrated in FIG. 8, each of the groups of LED chips 150includes a first LED chip 151, a second LED chip 153, and a third LEDchip 155 arrayed in a line along the lengthwise direction and emittinglight at different wavelengths. In this embodiment, the first LED chip151 may be a red LED chip that emits light of an approximately redwavelength band when power is applied thereto, the second LED chip 153may be a green LED chip that emits light of a green wavelength band whenpower is applied thereto, and the third LED chip 155 may be a blue LEDchip that emits light of a blue wavelength band when power is appliedthereto.

The first LED chip 151 has a 1st first electrode 151 a bonded to thefirst individual electrode pad 131 through a 1st first bump 171 a and a1st second electrode 151 b bonded to the first branch 136 of the commonelectrode pad 139 through a 1st second bump 171 b at the side facing thesubstrate 110.

The second LED chip 153 has a 2nd first electrode 153 a bonded to thesecond individual electrode pad 132 through a 2nd first bump 173 a and a2nd second electrode 153 b bonded to the second branch 137 of the commonelectrode pad 139 through a 2nd second bump 173 b at the side facing thesubstrate 110.

The third LED chip 155 has a 3rd first electrode 155 a bonded to thethird individual electrode pad 133 through a 3rd first bump 175 a and a3rd second electrode 155 b bonded to the third branch 138 of the commonelectrode pad 139 through a 3rd second bump 175 b at the side facing thesubstrate 110.

All LED chips 151, 153, and 155 flip-chip bonded onto the substrate 110and arranged in a matrix are required to have substantially the sameheight in order to achieve intended color reproducibility. For thispurpose, all LED chips including the first LED chip 151, the second LEDchip 153, and the third LED chip 155 have the same height; the 1st firstelectrode 151 a formed at one lateral side of the first LED chip 151,the 2nd first electrode 153 a formed at one lateral side of the secondLED chip 153, and the 3rd first electrode 155 a formed at one lateralside of the third LED chip 155 have the same height on the sameimaginary straight line; and the 1st first electrode 151 a formed at theother lateral side of the first LED chip 151, the 2nd first electrode153 a formed at the other lateral side of the second LED chip 153, andthe 3rd first electrode 155 a formed at the other lateral side of thethird LED chip 155 have substantially the same height on the sameimaginary straight line.

The 1st first bump 171 a, the 2nd first bump 173 a, and the 3rd firstbump 175 a have the same first height when finally compressed, and the1st second bump 171 b, the 2nd second bump 173 b, and the 3rd secondbump 175 b have the same second height when finally compressed. Thefirst height may be determined to be different from the second height soas to compensate for the difference in the height of the first LED chip151, the second LED chip 153, and the third LED chip 155 stepped witheach other.

The step of each of the LED chips 151, 153, and 155 may be formed byetching an epilayer structure including a laminate structure of a lighttransmission growth substrate (particularly, a sapphire substrate), afirst conductive semiconductor layer, an active layer, and a secondconductive semiconductor layer to remove some areas of the downwardlyexposed second conductive semiconductor layer and the active layeradjacent thereto so that the first conductive semiconductor layer isexposed downward.

The difference in the height of the LED chips caused by etching can bereduced when the two electrodes of each LED chip are designed to havedifferent heights, but the presence of fine steps is inevitable. Forthis reason, the heights of the 1st first bump 171 a, the 2nd first bump173 a, and the 3rd first bump 175 a when they are not compressed aremade identical to the heights of the 1st second bump 171 b, the 2ndsecond bump 173 b, and the 3rd second bump 175 b when they are notcompressed. However, the first height of the 1st first bump 171 a, the2nd first bump 173 a, and the 3rd first bump 175 a when finallycompressed may be made different from the heights of the 1st second bump171 b, the 2nd second bump 173 b, and the 3rd second bump 175 b whenfinally compressed so as to compensate for the steps of the LED chipstaking into consideration the heights of the electrodes of the LEDchips.

When compressed to the first height, the 1st first bump 171 a, the 2ndfirst bump 173 a, and the 3rd first bump 175 a have bonding areascorresponding to 50-80% of the upper surface areas of the wide portionsof the first individual electrode pad 131, the second individualelectrode pad 132, and the third individual electrode pad 133,respectively. When finally compressed, the 1st second bump 171 b, the2nd second bump 173 b, and the 3rd second bump 175 b have bonding areascorresponding to 50-80% of the upper surface areas of the first branch136, the second branch 137, and the third branch 138, respectively.

When the bump 171 a, 173 a, 175 a, 171 b, 173 b or 175 b has a bondingarea corresponding to 50-80% of the upper surface area of thecorresponding individual electrode pad or the corresponding branch ofthe common electrode pad, the bump is prevented from coming into contactwith the adjacent bump, which reduces the danger of shorting. That is tosay, also in the case where the bonding area of the bump 171 a, 173 a,175 a, 171 b, 173 b or 175 b is predetermined, there is a possibilitythat the bump may be inclined to the adjacent bump by very smalldistortion or slipping in a practical process. In contrast, when thebonding area of the bump 171 a, 173 a, 175 a, 171 b, 173 b or 175 b islimited to ≤80% defined above, the bump does not come into contact withthe adjacent bump, which prevents the occurrence of shorting caused byslipping or distortion in a practical process. If the bonding area is<50%, reliable bonding is not ensured. Accordingly, the area where thebump is finally bonded to the corresponding electrode pad or branch ispreferably limited to at least 50%.

The first branch 136, the second branch 137, and the third branch 138are portions of the common electrode pad and define the bonding areas ofthe 1st second bump 171 b, the 2nd second bump 173 b, and the 3rd secondbump 175 b, respectively, to fundamentally prevent the occurrence ofshorting caused by distortion or slipping of the 1st second bump 171 b,the 2nd second bump 173 b, and the 3rd second bump 175 b duringflip-chip bonding. In other words, without the first, second, and thirdbranches 136, 137, and 138 separated from each other by the recesses atone side of the common electrode pad 139, the 1st second bump 171 b, the2nd second bump 173 b or the 3rd second bump 175 b may slip or may bedistorted on the common electrode pad during flip-chip bonding, causingproblems such as shorting. In contrast, when the first, second, andthird branches 136, 137, and 138 are provided, the 1st second bump 171b, the 2nd second bump 173 b, and the 3rd second bump 175 b slip or aredistorted within the limited areas on the common electrode pad, thuspreventing problems such as shorting.

The plurality of LED chips 151, 153, and 155 flip-chip bonded to thesubstrate 110 are grouped into a plurality of groups of LED chips 150arranged in a matrix. Each of the groups of LED chips 150 consists ofthe first LED chip 151, the second LED chip 153, and the third LED chip155 arranged at uniform intervals along the lengthwise direction. The1st first electrode 151 a, the 2nd first electrode 153 a, and the 3rdfirst electrode 155 a are electrically connected to the first, second,and third individual electrode pads 131, 132, and 133, respectively, andthe 1st second electrode 151 b, the 2nd second electrode 153 b, and the3rd second electrode 155 b are electrically connected to the commonelectrode pad 139. This electrical connection enables individual controlof the first LED chip 151, the second LED chip 153, and the third LEDchip 155 belonging to each LED chip group 150.

The LED chip groups 150 arranged in a matrix on the substrate 110 havethe same intervals in the widthwise and lengthwise directions. Inaddition, the intervals of the LED chip groups 150 in the widthwisedirection are identical to those in the lengthwise direction. To thisend, the electrode patterns 130 arranged in a matrix on the substrate110 are designed to have the same intervals in the widthwise andlengthwise so as to correspond to the LED chips. In addition, theintervals of the electrode patterns 130 in the widthwise direction arepreferably designed to be identical to those in the lengthwisedirection.

The interval between the first LED chip 151 and the second LED chip 153is the same as that between the second LED chip 153 and the third LEDchip 155 and is preferably from 0.3 to 1.5 mm. For example, the chipsmay be designed to have intervals of approximately 0.75 mm for an FHDLED module and 0.375 mm for a UHD LED module.

Referring now to FIG. 10, an LED module further including additionalelements compared to the foregoing embodiments will be explained.

FIG. 10 illustrates an LED module 100′ including a barrier 190 formed onthe substrate 110 to prevent light interference between the adjacent LEDchips 151, 153, and 155 in each LED chip group 150. In this embodiment,the barrier 190 includes first light reflective walls 191 formed betweenthe adjacent first and second LED chips 151 and 153 and between theadjacent second and third LED chips 155 and second light reflectivewalls 193 formed between the LED chip groups 150 adjacent to each otherin the lengthwise direction and between the LED chip groups 150 adjacentto each other in the widthwise direction.

The first light reflective walls 191 and the second light reflectivewalls 193 can prevent the interference of light from the adjacent LEDchips in the LED chip group 150 and the interference of light from theadjacent LED chips of the adjacent two LED chip groups 150,respectively. This interference causes loss of quality of light from theLED chips. Instead of the barrier 190 including the light reflectivewalls, barriers capable of absorbing light from the LED chips may beformed between the LED chips 151, 153, and 155 or between the LED chipgroups 150 to prevent the interference of light from the LED chips.

Referring to FIGS. 11 and 12, an explanation will be given regarding amethod for fabricating the LED module illustrated in FIGS. 7 to 9.

Referring to FIGS. 7, 8, 9, 11, and 12, a method for fabricating the LEDmodule according to the present invention includes: forming theelectrode patterns 130, each of which includes the first individualelectrode pad 131, the second individual electrode pad 132, the thirdindividual electrode pad 133, and the common electrode pad 139, in amatrix on the substrate 110, as illustrated in FIG. 7 (S01); loading the1st first bumps 171 a, the 2nd first bumps 173 a, and the 3rd firstbumps 175 a on the first individual electrode pads 131, the secondindividual electrode pads 132, and the third individual electrode pads133, respectively, and loading the 1st second bumps 171 b, the 2ndsecond bumps 173 b, and the 3rd second bumps 175 b on the first branches136, the second branches 137, and the third branches 138 of the commonelectrode pads 139, respectively, as illustrated in FIG. 8(S02); andcollectively mounting the plurality of LED chips including the first,second, and third LED chips 151, 153, and 155 having differentwavelength characteristics to predetermined heights on the substrate 110such that the groups of LED chips 150 are arranged in a matrix on thesubstrate 110, as illustrated in FIG. 11(S03).

Particularly, the step of arranging the groups of LED chips 150 in amatrix on the substrate 110 (S03) includes transferring the plurality ofLED chips 151, 153, and 155 arranged in a predetermined matrix to asupport B by a roll-to-roll transfer printing technique and mounting theLED chips without changing their original matrix on the substrate 110.

As explained in detail below, the roll-to-roll transfer printingtechnique for the transfer of the LED chips 151, 153, and 155 uses abonding carrier, a pick-up roll 40 adapted to pressurize the bondingcarrier at the time when the LED chips 151, 153, and 155 are picked up,and a chip placing roll 50 adapted to pressurize the LED chips 151, 153,and 155 at the time when the LED chips 151, 153, and 155 are mounted onthe substrate 110. The pick-up roll 40 and the chip placing roll 50operate in cooperation with each other against the bonding carrier atchip pick-up and chip mounting positions so that the LED chips 151, 153,and 155 can be transferred to and mounted on the substrate 110 from thesupport B without changing their original matrix. The bonding carrier isan element that is adhesive enough to bond and carry the plurality ofLED chips 151, 153, and 155. The pick-up roll 40 is an element adaptedto pressurize the bonding carrier against the LED chips such that theLED chips located on the support B are bonded to the bonding carrier.The chip placing roll 50 is an element adapted to pressurize theplurality of LED chips 151, 153, and 155 against the substrate 110without changing their original matrix bonded to the bonding carrier. Inaddition to these elements, additional means may be used to partiallyweaken the adhesive strength of the bonding carrier before pick-up ofthe LED chips using the pick-up roll 40 or to weaken the adhesivestrength of the bonding carrier as a whole before mounting of the LEDchips 151, 153, and 155 on the substrate 110 using the chip placing roll50.

As briefly mentioned above, the step of arranging the groups of LEDchips 150 in a matrix on the substrate 110 (S03) includes: bonding theLED chips including the first LED chips 151 having the 1st firstelectrodes 151 a and the 1st second electrodes 151 b, the second LEDchips 153 having the 2nd first electrodes 153 a and the 2nd secondelectrodes 153 b, and the third LED chips 155 having the 3rd firstelectrodes 155 a and the 3rd second electrodes 155 b in a matrix to abonding carrier 10′ having some areas whose adhesive strength isweakened (S1, S3); moving the bonding carrier such that the 1st firstelectrodes 151 a, the 2nd first electrodes 153 a, and the 3rd firstelectrodes 155 a face the 1st first bumps 171 a, the 2nd first bumps 173a, and the 3rd first bumps 175 a, respectively, and the 1st secondelectrodes 151 b, the 2nd second electrodes 153 b, and the 3rd secondelectrodes 155 b face the 1st second bumps 171 b, the 2nd second bumps173 b, and the 3rd second bumps 175 b, respectively (S4); andpressurizing the first LED chips 151, the second LED chips 153, and thethird LED chips 155 to a predetermined pressure such that the 1st secondbumps 171 b, the 2nd second bumps 173 b, and the 3rd second bumps 175 bare compressed to a first height and the 1st second bumps 171 b, the 2ndsecond bumps 173 b, and the 3rd second bumps 175 b are compressed to asecond height (chip placing down, S7).

The steps of bonding the LED chips in a matrix (S1 and S3) includeprimarily exposing the bonding carrier 10 to produce a bonding carrier10′ consisting of areas whose adhesive strength is weakened and areaswhose adhesive strength remains unweakened (S1) and pressurizing theareas whose adhesive strength remains unweakened against the LED chips151, 153, and 155 using a rolling pick-up roll 40 to bond the LED chips151, 153, and 155 to the bonding carrier 10′ (S3).

First, before the LED chips 151, 153, and 155 are attached in a matrixto the bonding carrier, the LED chips 151, 153, and 155 are arranged ina matrix on the support B. Here, the LED chips arranged in a matrix havethe same intervals in the widthwise intervals and lengthwise directionsas a matrix of the LED chips 151, 153, and 155 to be attached to thebonding carrier 10′ having areas whose adhesive strength is weakened anda matrix of the LED chips 151, 153, and 155 to be mounted on thesubstrate 110 in the subsequent processes. Before attachment of the LEDchips 151, 153, and 155 in a matrix to the bonding carrier 10, notreatment is carried out to weaken the adhesive strength of the bondingcarrier 10. The bonding carrier 10 is located on the LED chips 151, 153,and 155, and a photomask 20 and an exposure device 30 for primaryexposure of the bonding carrier 10 may be arranged on the bondingcarrier 10.

It is necessary to weaken the adhesive strength of areas of the bondingcarrier that do not correspond to the LED chips 151, 153, and 155. Tothis end, in S1, UV light from the exposure device 30 is irradiated onlyonto areas of the bonding carrier not corresponding to the LED chips151, 153, and 155 through through-holes of the photomask 20. The UVlight acts on the bonding carrier 10 at a constant temperature of 200°C. or less to weaken the adhesive strength of some areas of the bondingcarrier 10. As a result, areas whose adhesive strength is weakened areintermittently formed in the bonding carrier 10′.

In S3, the LED chips 151, 153, and 155 are attached to the areas of thebonding carrier 10 whose adhesive strength remains unweakened. Thepick-up roll 40 arranged on the bonding carrier 10′ having the areaswhose adhesive strength is weakened pressurizes the bonding carrier 10′against the LED chips 151, 153, and 155. At this time, each of thebonding carrier 10′ and the support B is maintained fixed and thepick-up roll 40 rolls to sequentially pressurize all areas of thebonding carrier 10′ against the LED chips 151, 153, and 155. By themovement of the pick-up roll 40, the LED chips 151, 153, and 155 can beprevented from slipping off their set positions relative to the bondingcarrier 10′ and being adsorbed to the bonding carrier 10′.

It is preferred to secondarily expose the bonding carrier 10′ justbefore S7. When secondarily exposed, the bonding carrier 10′ having someareas whose adhesive strength is weakened is converted into a bondingcarrier 10″ whose adhesive strength is weakened as a whole. In S7, thechip placing roll 50 pressurizes the LED chips 151, 153, and 155 to apredetermined pressure while rolling on the bonding carrier 10″ whoseadhesive strength is weakened as a whole. Subsequently, the bondingcarrier 10″ whose adhesive strength is weakened as a whole is detachedfrom the LED chips 151, 153, and 155 (S9).

The pressurization by the rolling of the chip placing roll 50 allows theLED chips 151, 153, and 155 bonded to the bonding carrier 10″ to besequentially attached to the substrate 110. Here, the bumps can beheated above a predetermined temperature. By the rolling placing roll50, the LED chips 151, 153, and 155 are reliably mounted atpredetermined heights and intervals at set positions of the substrate110.

According to this method, the LED chips 151, 153, and 155 can be mountedon the substrate 110 in a short time. In addition, the mounted LED chips151, 153, and 155 emitting light at different wavelengths can be groupedto correspond to pixels. These groups can be arranged in a matrix tofabricate the LED module. The LED chips arranged in a matrix arecollectively bonded to the bonding carrier and are then transferred toand mounted on the substrate 110 without changing their original matrix,enabling precise control over the alignment and interval of the LEDchips 151, 153, and 155.

The LED module having arrays of the LED chips emitting light atdifferent wavelengths is not limited to the constitutional andoperational modes of the foregoing embodiments. All or some of theembodiments are selectively combined for various modifications.

[Fabrication of Third Type LED Module]

Referring to FIG. 13, a method for fabricating an LED module includesproducing a chip-on-film including an adhesive chip retaining film and aplurality of LED chips bonded onto the chip retaining film (S1),transferring the LED chips from the adhesive chip retaining film to acarrier tape (chip pick-up, S2), moving the carrier tape to move the LEDchips (S3), and transferring the LED chips from the carrier tape to asubstrate (particularly, a circuit board or an AM substrate) (chipplacing S4).

S2, S3, and S4 are sequentially carried out after feeding of LED chipsinto a chip array system, which will be presented hereinafter. S1 iscarried out before a chip array process in the chip array system.

S1 includes attaching a plurality of flip-bonded LED chips 1 onto anadhesive chip retaining film 2 in a matrix, as illustrated in FIGS. 14,and 15. Each of the LED chips 1 includes a light emitting semiconductorstructure 10 having two bottom areas 1 a and 1 b, which have differentstep heights, a first conductive electrode pad 112 disposed in the area1 a, and a second conductive electrode pad 142 disposed in area 1 b,respectively. The light emitting semiconductor structure 10 includes abase substrate 11, a first conductive semiconductor layer 12, an activelayer 13, and a second conductive semiconductor layer 14. The basesubstrate 11 may be a sapphire growth substrate on which an epilayerincluding the first conductive semiconductor layer 12, the active layer13, and the second conductive semiconductor layer 14 is grown.Alternatively, the base substrate 11 may be a support substrate to whichepilayers are attached. The exposed areas 1 a and 1 b are formed on thebottom surfaces of the first conductive semiconductor layer 12 and thesecond conductive semiconductor layer 14, respectively. The firstconductive electrode pad 112 is formed in the exposed area 1 a on thefirst conductive semiconductor layer 12 and the second conductiveelectrode pad 142 is formed in the exposed area 1 b on the secondconductive semiconductor layer 14.

The plurality of LED chips 1 are arrayed such that the first conductiveelectrode pads 112 and the second conductive electrode pads 142 aredirected downward and are bonded to bonding areas of the chip retainingfilm 2. The chip retaining film 2 is designed to lose its adhesivenesswhen irradiated with UV light. Due to this design, UV light irradiatedonto particular areas of the chip retaining film 2 relatively weakensthe adhesiveness of the particular areas.

On the other hand, S2, S3, and S4 may be sequentially carried out in achip array system including a carrier tape 3, a pick-up stage 210, aphotomask 220, a UV scan set 230 for film exposure, a pick-up roller240, a placing roller 260, and a UV light source 250 for tape exposure,as illustrated in FIGS. 14 to 18.

Referring first to FIG. 14, in S1, a chip-on-film c 1 including anadhesive chip retaining film 2 and a plurality of LED chips 1 arrayedthereon is arranged on a pick-up stage 210. Here, the pitch in the LEDchip arrays on the chip retaining film 2 is determined to be differentfrom the desired pitch in the LED chip arrays on a target substrate 5.That is to say, the ratio of the desired number of the LED chips in theLED chip arrays (i.e. the number of pairs of electrodes) on a targetsubstrate 5 (see FIGS. 17 and 18) to the number of the LED chips in theLED chip arrays on the chip retaining film 2 is defined as 1:n (where nis a natural number equal to or greater than 1). Thus, the number of theLED chips 1 attached to the chip retaining film 2 is n times larger thanthe number of the LED chips 1 arranged on the substrate 5 (see FIGS. 17and 18). Some of the LED chips 1 attached to the chip retaining film 2can be selectively detached from the chip retaining film 2 and attachedonto the substrate 5. Here, the substrate 5 may be a substrate formedwith an electric circuit (more specifically, an active matrixsubstrate).

After the chip-on-film c 1 is arranged on a UV transmitting upper plate211 of the pick-up stage 210, the adhesive strength of areas of the chipretaining film 2 where particular LED chips 1 are attached is weakened.As a result, only the selected LED chips 1 can be picked up from thechip retaining film 2. To this end, a photomask 220 and a UV scan set230 for film exposure are used that underlie the UV transmitting upperplate 211. A plurality of UV transmitting windows 222 are formed in thephotomask 220.

The UV scan set 230 for film exposure includes a UV light source 234 forfilm exposure and a light source carrier 232 adapted to move the UVlight source 234 to a position corresponding to a particular one of theUV light-transmitting windows 222. The UV light source 234 located underthe particular UV light-transmitting window 222 irradiates UV onto aparticular area of the chip retaining film 2 attached with theparticular LED chip 1. This UV exposure weakens the adhesive strength ofthe chip retaining film 2 to the particular LED chip 1. The plurality ofUV light-transmitting windows 222 may be arranged in the X-axisdirection and the Y-axis direction orthogonal thereto. The light sourcecarrier 232 may move the UV light source 234 in the X- and Y-axisdirections.

The adhesive strength of the chip retaining film 2 to the selected LEDchip 1 is weakened, and simultaneously, the pick-up roller 240pressurizes the carrier tape 3 against the selected LED chip 1 to attachthe selected LED chip 1 to the carrier tape 3 while rolling in onedirection. The rolling of the pick-up roller 240 can be accomplished bythe translation of the pick-up roller 240. Alternatively, the rolling ofthe pick-up roller 240 may be accomplished by the translation of thestage 200 while the pick-up roller 240 rolls in place.

The adhesive strength of one area of the chip retaining film 2 exposedto UV light is lower than that of the carrier tape 3 because the arealoses its adhesive strength by UV exposure. Due to the weakened adhesivestrength, the selected LED chip 1 is attached to the carrier tape 3.Although all LED chips 1 in the particular LED chip array on the chipretaining film 2 are brought into contact with the carrier tape 3 bypressurization of the rolling pick-up roller 240, the LED chips 1 in theareas of the chip retaining film 2 whose adhesive strength is weakenedby UV exposure (i.e. only the selected LED chips 1) are attached to thecarrier tape 3, and the other LED chips 1 (i.e. the LED chips 1 in theareas unexposed to UV light and whose adhesive strength is not weakened)remain on the chip retaining film 2. The LED chips 1 remaining on thechip retaining film 2 can be detached from the chip retaining film 2 inother subsequent repeated chip pick-up processing steps.

The pick-up roller 240 is provided with a flexible blanket 242 on theouter circumference of a roller body 241 coupled to a shaft. Theprovision of the blanket 242 allows the LED chips 1 to be betterattached to the transfer tape 3 and protects the LED chips 1 from damagecaused by pressurization.

The LED chips 1 to be picked up may be selected in such a manner thatthe UV light source 234, the UV light-transmitting window 222 of thephotomask 220, and the area where the LED chip 1 to be picked up isattached are allowed to lie on the same imaginary vertical line throughthe movement of the UV light source 234 in the X- or Y-axis direction bythe light source carrier 232. Thus, the pitch of the LED chips 1 in theparticular LED chip array on the chip retaining film 2 is different fromthat of pairs of electrodes on the substrate mounted with the LED chips,and as a result, the ratio of the number of the LED chips in theparticular LED chip array on the chip retaining film 2 to the number ofpairs of electrodes on the substrate (i.e. the desired number of the LEDchips in the LED chip array on the substrate) is n:1. In this case, thepick-up and placing down may be repeated n times.

Referring next to FIG. 16, the carrier tape on which the LED chips 1 arepicked up is moved in one direction to move the chips (S3). The carriertape 3 has a predetermined adhesive strength to the LED chips 1. Thecarrier tape 3 together with the LED chips 1 attached and held thereonis moved by suitable moving means, for example, a feeding roller and aguiding roller. The UV light source 250 may be arranged in the middle ofthe flow path of the carrier tape 3 or around the placing roller 260.The UV light source 250 irradiates UV light onto the carrier tape 3 toweaken the adhesive strength of the carrier tape 3 as a whole.

Referring to FIGS. 17 and 18, one area of the carrier tape 3 attachedwith the LED chips 1 is moved between the placing roller 260 and thesubstrate 5 formed with pairs of bumps 5 a and 5 b. Here, the adhesivestrength of the carrier tape 3 exposed to UV light is lower than that ofan adhesive loaded on the substrate 5, more specifically on the pairs ofbumps 5 a and 5 b. The placing roller 260 rolls and pressurizes the LEDchips 1 attached to the carrier tape 3 against the substrate 5, morespecifically against the pairs of bumps 5 a and 5 b on the substrate 5,to attach the corresponding LED chips 1 onto the substrate 5. Theplacing roller 260 may be provided with a flexible blanket 262 on theouter circumference of a roller body 261 coupled to a shaft. Theprovision of the blanket allows the LED chips 1 to be better placed downduring rolling and can protect the LED chips 1 from damage caused bypressurization during rolling. The LED chips 1 placed down on thesubstrate 5 can be bonded onto the substrate 5 by a reflow solderingprocess.

FIGS. 19, 20 a, and 20 b compare the pick-up of LED chips by totaltransfer with the pick-up of LED chips by selective transfer.

Referring to FIG. 19, according to the pick-up of LED chips by totaltransfer, the pick-up roller 240 sequentially pressurizes all LED chips1 in the corresponding array a1 on the chip retaining film 2, and as aresult, the LED chips 1 are transferred and attached without omission tothe carrier tape 3. In this case, the pitch of the LED chips 1 in thecorresponding array a1 on the chip retaining film 2 is the same as thepitch in the array on the substrate to which the LED chips 1 are to betransferred, and the ratio of the number of the LED chips arrayed on thechip retaining film 2 to the number of the LED chips arrayed on thesubstrate is 1:1.

In contrast, referring to FIGS. 20a and 20b , according to the pick-upof LED chips by selective transfer, the placing roller 260 pressurizesall LED chips 1 in the corresponding array a1 during rolling but onlythe LED chips 1 in the areas of the chip retaining film 2 whose adhesivestrength is weakened using the UV scan set 230 and the photomask 220 arepicked up on the carrier tape 3.

As illustrated in FIG. 20a , a one-time pick-up operation can also beperformed to pick up the LED chips 1 in one array a1. As illustrated inFIG. 20b , a one-time pick-up operation may also be performed tosimultaneously pick up the LED chips 1 in two or more arrays a1 and a2.Although illustrated, the present invention can be advantageously usedfor selective pick-up of the LED chips 1 attached to the chip retainingfilm 2 in a complex pattern as well as in a matrix.

What is claimed is:
 1. A method for fabricating a display module,comprising: forming electrode patterns, each of which comprises a firstindividual electrode pad, a second individual electrode pad, a thirdindividual electrode pad, and a common electrode pad, in a matrix on asubstrate; loading 1st first bumps, 2nd first bumps, and 3rd first bumpson the first individual electrode pads, the second individual electrodepads, and the third individual electrode pads, respectively, and loading1st second bumps, 2nd second bumps, and 3rd second bumps on the commonelectrode pads; bonding LED chips comprising first LED chips having 1stfirst electrodes and 1st second electrodes, second LED chips having 2ndfirst electrodes and 2nd second electrodes, and third LED chips having3rd first electrodes and 3rd second electrodes in a matrix to a bondingcarrier; moving the bonding carrier such that the 1st first electrodes,the 2nd first electrodes, and the 3rd first electrodes face the 1stfirst bumps, the 2nd first bumps, and the 3rd first bumps, respectively,and the 1st second electrodes, the 2nd second electrodes, and the 3rdsecond electrodes face the 1st second bumps, the 2nd second bumps, andthe 3rd second bumps, respectively; and pressurizing the first LEDchips, the second LED chips, and the third LED chips to a predeterminedpressure such that the 1st second bumps, the 2nd second bumps, and the3rd second bumps are compressed to a first height and the 1st secondbumps, the 2nd second bumps, and the 3rd second bumps are compressed toa second height (chip placing down), wherein each of the electrodepatterns is formed such that the common electrode pad is aligned to afirst end line parallel to the lengthwise direction of the matrix, andthe first individual electrode pad, the second individual electrode pad,and the third individual electrode pad are aligned to a second end lineparallel to the first end line and are located between the first endline and the second end line; and the chip placing down is carried outsuch that the first LED chip, the second LED chip, and the third LEDchip are arrayed in a line along the lengthwise direction.
 2. The methodaccording to claim 1, wherein the common electrode pad comprisesrecesses depressed inwardly from the first end line and a first branch,a second branch, and a third branch, whose shapes are defined by therecesses.
 3. The method according to claim 1, wherein the bonding of theLED chips in a matrix to the bonding carrier comprises primarilyexposing the bonding carrier to form areas whose adhesive strength isweakened and areas whose adhesive strength remains unweakened on thebonding carrier and pressurizing the areas whose adhesive strengthremains unweakened against the LED chips using a rolling pick-up roll tobond the LED chips to the bonding carrier.
 4. The method according toclaim 3, wherein the bonding carrier is secondarily exposed to weakenthe adhesive strength of the bonding carrier as a whole before the chipplacing down, and the chip placing down is carried out such that a chipplacing roll pressurizes the LED chips to a predetermined pressure whilerolling on the bonding carrier.